There have been proposed electrodes of various structures and compositions for use in various types of electric devices, electronic devices and electrochemical devices. In particular, there is known, as one of such kinds of electrodes, a so-called “porous electrode” (cermet electrode) formed of a sintered mixture (cermet) produced by sintering an electrode metal material and a ceramic material.
As well known in the art, such porous electrodes formed of an electrode metal material and a ceramic material which have respective properties not only exhibit new properties not obtained by electrodes formed solely of the electrode metal material, but also exhibit various properties according to specific combinations of various electrode metal materials and ceramic materials. The porous electrodes are used in various gas sensor elements and other detecting devices for measuring oxides or combustible gases contained in combustion gases in incinerators and industrial furnaces or contained in exhaust emissions of vehicles, or used in various types of devices such as fuel cells, depending upon the specific properties of the porous electrode.
Such porous electrodes as used in NOx sensor elements, oxygen sensor elements and any other gas sensor elements, for example, take the form of porous thin films each having a multiplicity of pores formed therein, and are utilized as measuring electrodes (detecting electrodes) for measuring desired gas components such as NOx and oxygen of a measurement gas, as disclosed in Patent Document 1. In these porous electrodes used as the measuring electrodes used in such various kinds of gas sensor elements, the desired gas components in the measurement gas flow through the multiple pores and come into contact with the electrode metal material within the porous electrode structure. However, this porous electrode structure has a risk of suffering from such defects as described below.
Namely, the porous electrode used, for example, as the measuring electrode in the gas sensor elements, has repeated volumetric changes caused by expansion/contraction of the electrode metal material and ceramic material during repeated measurements of the desired gas components or due to a change of the temperature of the desired gas components. The electrode metal material and the ceramic material have different amounts of change of their volumes due to different ratios of the volumetric change caused by the temperature change, so that the porous electrode structure is subject to a stress generated therein. It is also noted that the electrode metal material has a volumetric change caused by its expansion/contraction due to oxidization/reduction of the electrode metal material by oxidizing components or reducing components contained in the measurement gas. This volumetric change of the electrode metal material also causes a stress to be generated in the electrode structure. Such stresses generated within the porous electrode structure give rise to a risk of cracking of the ceramic material bonded to the electrode metal material, or a risk of removal of the electrode structure as a whole from a solid electrolyte body, which may disable the porous electrode to serve its function.
Where the porous electrode is covered by a protective layer formed thereon to protect the porous electrode, the stresses generated as described above may cause cracking of the protective layer as well.
Patent Document 1: JP-2000-28576 A